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Truly Optimize FCC Gasoline Distillation

By Optimization Specialist Robert

Apr 26, 2016
 

Leverage cross-functional team-work to tackle optimization variables related to FCC gasoline properties.

 
 

Every great planning analyst started off as a process engineer.  So with confidence, I can state that the majority of process engineers do not really know how to set unit operating targets on their own. 

 

Through collaboration of refinery planning, process, and control engineers, a refinery can truly define optimal operating targets.  Whether you believe this or not, I will discuss several decision points around FCC gasoline stream optimization to support my view. 

 

If I were to give one piece of advice, I would suggest that every good refiner should always ask “what is the value of the alternative disposition?” in order to understand how to set appropriate targets.  Use this step-by-step guide of questions to properly frame the analysis:

 

  1. What is the value of the molecules in the stream above?
  2. What is the value of the molecules in the stream below?
  3. What upstream unit operations affect the stream value?
  4. What downstream unit operations affect the stream value?
  5. What unit specific operations affect the stream value?
  6. What product blending constraints affect the stream value?

 

Let’s start by focusing on a simplified PFD of the FCC distillation section.  Most FCC units have only two gasoline product streams coming off the unit – the Light Cat Naphtha (LCN) and Heavy Cat Naphtha (HCN).

 

Historically, many refiners combined both gasoline streams into a Mixed Cat Naphtha pool, but today’s regulatory environment and market economics drive different unit configurations and stream routings.

 

“what is the value of the

alternative disposition?”

 

 

 

Light Cat Naphtha

 

We will begin our analysis by focusing on the Light Cat Naphtha stream.  How many variables do you control related to the LCN stream?  If you are a process engineer worth your salt, you will say at least half a dozen for this one stream alone!

 

If this response surprises you, let’s work through my series of questions above to frame the analysis.

 

  1.

What is the value of the molecules in the stream above?

   

 

A sub-optimal cut between Olefins and LCN will either leave too many olefins in the LCN stream, or not enough.  The value of olefin stream above the LCN will be governed by downstream unit capabilities, as well as market conditions.  Is there an alky unit downstream of the FCC, and if so, does the Alky have capacity to take more feed? 

 

Also, consider the type of Alky downstream of the FCC.  If your refinery has a Sulfuric Alky, you have greater ability to cut deeper in the olefin draw to capture C5= olefins.  If your refinery has a HF Alky, olefin recovery will be limited by acid strength concerns. 

 

As for market conditions, you should next consider if it is summer or winter.  During the summer time, Alkylation economics strength as isobutane prices deflate and the value of octane strengthens. 

 

  2. What is the value of the molecules in the stream below?
   

 

Below the LCN stream is HCN.  When gasoline prices are above that of distillate prices, some refiners may be indifferent to the LCN/HCN cutpoint distillation as both streams end up in gasoline product.  When distillate prices are above that of gasoline, most refiners try to minimize the LCN endpoint to maximize distillate production

 

  3. What upstream unit operations affect the stream value?
   

 

With increasing regulations that minimize gasoline sulfur content, FCC gasoline sulfur continues to drive greater scrutiny.  Refiners that have FCC feed Hydrotreaters have the ability to manage FCC gasoline sulfur by optimizing Hydrotreater severity. 

 

Alternatively, sub-optimal operation of the FCC feed Hydrotreater may result in lower crude sulfur requirements, or sub-optimal Light Cat Naphtha endpoints to manage gasoline pool sulfur.  The point is, your degrees of freedom on LCN endpoint or LCN RVP may be influenced by factors upstream of the FCC.

 

  4. What downstream unit operations affect the stream value?
   

 

What is the end destination of the LCN stream in your refinery?  Does LCN go to a Gasoline Hydrotreater (GHT) and ultimately to the gasoline blend pool?  Or alternatively, does LCN get routed to a Selective Hydrogenation Unit (SHU) and then feed a NHT/Reformer? 

 

The ability to minimize olefin saturation in a downstream Hydrotreater has an impact on your refinery hydrogen and octane balance.   If refiners leave a high amount of olefins in the LCN stream, high post-treater severity will increase hydrogen consumption and result in lower stream octane values.  If GHT/SHU severity is not a consideration for how you manage the Olefin/LCN interface, you should think twice about this.

 

  5. What unit specific operations affect the stream value?
   

 

So what about FCC operations in itself?  I’m sure that this is one of the first items that you consider when deciding how to optimize your product cutpoints.  Or is it?

 

Every refinery sets FCC Riser targets based on refinery configuration and market economics.  However, many refineries fail to recognize the importance of managing product distillation targets as a function of riser adjustments. 

 

While riser targets may be primarily influenced by volume expansion, discounted LPG value, and gasoline vs distillate economics, subtle constraints such as olefin content and octane value will be influenced by a combination of riser and distillation targets. Similar to FCC Riser, cat-to-oil ratio affects product mix, thus distillation strategies.

 

  6. What product blending constraints affect the stream value?
   

 

After considering the 5 points above, a prudent refiner must now consider product blend constraints to properly define stream distillation targets.  For LCN distillation optimization, refiners need to primarily consider constraints for gasoline blending. 

 

As LCN is often routed to gasoline, we should understand flexibility to manage olefins, sulfur, and aromatics.  Depending on your region, each one of these may surface as a hard blending constraint, by which you may need to change stream distillation to manage.

 

 

Heavy Cat Naphtha

 

Similar to our analysis on LCN, Heavy Cat Naphtha distillation has just as many considerations as well.  As many of the LCN/HCN distillation variables were discussed above, I’ll now cover HCN/LCO and HCN routing factors below.

 


1.
What is the value of the molecules in the stream above?



See LCN discussion above



2.
What is the value of the molecules in the stream below?



Many refiners have the ability to route LCO to Diesel Hydrotreaters or Hydrocrackers.  This provides great flexibility as FCC main fractionator operations enable refiners another handle to increase diesel production. 

 

When diesel prices are higher than that of gasoline, refiners can minimize HCN endpoint to a diesel flash limit.  If LCO is routed to a Hydrocracker instead of a Hydrotreater, HCN endpoint can be adjusted to a jet flash limit. 

 

Oftentimes, HCN endpoint can also be used to optimize fuel oil blending if LCO is used as a cutter.  When distillate prices are higher than that of gasoline, it can also prove economic to minimize HCN endpoint so that the LCO stream has better viscosity cutting efficiency. 



3.
What upstream unit operations affect the stream value?



Similar to the LCN stream, HCN product sulfur can be influenced by decisions made upstream of the FCC.  If HCN is routed to gasoline blending, the stream endpoint may be sub-optimally adjusted if the sulfur content cannot be managed through desulfurization or feedstock optimization efforts.


4.
What downstream unit operations affect the stream value?



FCC Gasoline Post-treater efficiency may have an effect on HCN distillation targets.  If gasoline sulfur specs limit your refinery, you may need to reduce HCN EP if your refinery is desulfurization limited. 

 

If HCN can be routed to a Jet Hydrotreater, make-up hydrogen constraints may limit the HCN volume or distillation.  As HCN contains more unsaturated molecules than straight-run feedstocks, hydrogen consumption will increase significantly for HCN. 



5.
What unit specific operations affect the stream value?



Often-times, draw constraints on a fractionator can be managed differently by adjusting unit yield profiles.  Most engineers intuitively adjust distillation parameters to manage draw constraints; however, many engineers neglect the handle to using reactor yields.  While every FCC unit is built differently, don’t forget that unit severity or catalyst circulation are dynamic variables that can be used as well. 


6.
What product blending constraints affect the stream value?



As Heavy Cat Naphtha can be routed to gasoline, jet, or diesel, all of the blend constraints for each product pool need to be considered when adjusting HCN distillation.

 

  • When routing HCN to gasoline, use HCN endpoint to adjust gasoline sulfur, endpoint, and aromatics. 
  • When routing HCN to jet, use HCN IBP to meet jet flash.  Accordingly, use HCN endpoint to manage jet freeze and smoke point.
  • When routing HCN to diesel, use IBP to manage diesel flash. 


 

Now that we’ve covered half-a-dozen considerations for each FCC gasoline product stream, you can see how proper optimization requires team-work to solve. 

 

A great process engineer understands how to capture value through flexibility of unit operations.  A great refinery economics provides direction on where market opportunities exist.  Combine the efforts of both, and unleash the full potential of value in FCC gasoline distillation.

 
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  • Shiraz Daud :   I wud like to see the pfd. so it would be nice help if u may share it with me "the pfd for the production of gasoline by the HF Alkylation Process"

    Nov 12, 2013

  • Hassan Abed Al-Kareem :   so valuable article...thanks a lot

    Nov 12, 2013

  • Kevin Proops :   A great process engineer might also understand that changing LCN-HCN split will affect LPG recovery in the primary abosrber, unless debutanized gasoline can be recycled. Less light cat naphtha will mean less absorption of light ends in the reflux drum an

    Nov 12, 2013

  • Sunil Raj Saxena :   Good Article. I would like to add that this applies to any Refinery Unit. Sub-optimal or Optimal are relative terms. The bottom line is that the Unit margin has to be maximized and this can be done along with the planner (economics guy) who is keeping an eye on the market & prices and can therefore guide the process engineer as to which stream to maximize and the scheduler/blender who can tell what are his contraints in blending the streams.

    May 02, 2016

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